Groundwater has always played an important role in human history. Groundwater contamination has been a subject of intensive investigations since the mid-1980s. Contaminants in water environments undergo transformations and changes in concentration resulting from physical, chemical, and/or biological processes. A capability to understand and model these processes is at the core of water-quality management. It has long been appreciated that numerical models can enhance fundamental understanding of coupled physical and biogeochemical processes in geologic media as well as enable quantification of performance and risk assessment for engineering applications. Consideration of equilibrium and kinetic chemistry, thermal transport, and hydrologic transport and interactions between fluid flow, heat flow, and reactive transport is necessary to represent the complexity of real systems.